The pre-systemd behavior is that the order is not really completely predictable. First of all, DNS queries are usually made with UDP. There is no ack or retran

The pre-systemd behavior is that the order is not really completely predictable. First of all, DNS queries are usually made with UDP. There is no ack or retransmission with UDP, and the network stack is free to drop messages if necessary, such as if a queue fills up or the network is congested. (Hence it has the nickname "Unreliable Datagram Protocol".) Second, "man resolv.conf" has this to say about what happens when you have multiple DNS servers listed: "The algorithm used is to try a name server, and if the query times out, try the next, until out of name servers, then repeat trying all the name servers until a maximum number of retries are made." Consider what happens when you combine those two together. You roll the dice and send a UDP request to the first server. Then it rolls the dice by sending a UDP response to you. If that doesn't work, you send to the second server. If that doesn't work, then you try the first server again. TLDR: In the case of lost packets, which you must assume happen because it's UDP, it alternates between servers. The only thing that's guaranteed is which one comes up in the rotation first.